Using Roundcube, you can delete all messages in a mailbox or specific messages of your choosing.

When you click on the "Delete" button on the screen after selecting the e-mails you want to delete, these e-mails will be moved to the "Trash" folder.

You must also delete the e-mails from the "Trash" folder that you want to completely delete from your account. You can reach the "Trash" folder by clicking the "Trash" button in the "Mail" tab in the main menu. When you select the e-mails in the "Trash" folder that you want to delete completely and click the "Delete" button, these e-mails will be deleted directly without being moved to another folder.

You can have your e-mails sent to the "Trash" folder automatically deleted when you log out. To do this, click the "Server Settings" button in the "Settings" tab in the main menu, select the "Clear Trash on logout" option in the "Maintenance" section and save it. E-mails that you delete from your Inbox or other folders while you're signed in and that have been moved to Trash will be automatically deleted from Trash when you sign out.